1. Field of the Invention
The present invention relates to a high-density magneto-optical recording medium and a method of reproducing information recorded on such a medium.
2. Description of the Related Art
In regard to a magneto-optical disk known as a high-density recording medium, it is currently demanded to achieve a further enhanced density in accordance with increase in the amount of information. Although a higher density can be realized by reducing the space between record marks, recording and reproduction of such marks are limited by the size of a laser beam (beam spot) on the medium. When the space is so set that merely a single record mark is existent within the beam spot, an output waveform corresponding to "1" or "0" determined by the presence or absence of a record mark can be observed as a reproduced signal.
However, if the space is so narrowed that several record marks are existent within the beam spot, no change is caused in the reproduced output regardless of any movement of the beam spot on the medium, so that the output waveform is rendered linear to consequently raise a problem that the presence or absence of any record mark is not discriminable. The beam spot may be dimensionally reduced for the purpose of attaining proper reproduction of small record marks having a period smaller than the size of the beam spot, but the dimensions of the beam spot are restricted by the wavelength .lambda. of a light beam source and the numerical aperture NA of an objective lens, so that it is impossible to sufficiently reduce the dimensions of the beam spot.
Recently, there is proposed a novel reproduction method which utilizes an existing optical system without any modification and reproduces record marks smaller than the size of a beam spot by the use of a magnetically induced superresolution medium (hereinafter referred to as MSR medium). This method is effective to enhance the resolution of reproduction by masking other marks during reproduction of one mark within the beam spot. Therefore, at least a mask layer or a reproducing layer is required, in addition to a recording layer, in the MSR medium for concealing all other marks so that only one mark is reproduced in a signal reproduction mode.
Hereinafter a brief description will be given on the MSR medium and a method of recording information on and reproducing the same from such medium. FIG. 1 show the structure of the MSR medium disclosed in Jpn. J. Appl. Phys. Vol. 31 (1992) pp. 568-575 part 1. No. 2B, February 1992, and the positional relationship between record marks and a beam spot on a record track. On an unshown transparent substrate, there are successively formed a magnetic reproducing layer 2, a magnetic switch layer 3 and a magnetic recording layer 4 in this order. Data are recorded in the recording layer 4 by the directions of magnetization, and record marks 6 are formed on the record track 5 at a pitch smaller than the diameter of the spot of an irradiated laser beam.
The reproducing layer 2 is composed of GdFeCo, and its Curie temperature is higher than 300.degree. C. Meanwhile the switch layer 3 is composed of GdFeCoAl, and its Curie temperature is approximately 140.degree. C. And the recording layer 4 is composed of GdFeCo, and its Curie temperature is approximately 250.degree. C. In the case of a magneto-optical recording medium, the area heated beyond a Curie temperature Tc is dimensionally reducible to be smaller than the diameter of a laser beam spot by controlling the power of the laser beam in a recording mode, whereby it is made not so difficult to form small record marks.
Now a description will be given on a method of reproducing information recorded on the medium. At a room temperature, the magnetization of the reproducing layer is rendered directionally coincident with that of the recording layer 4 by switched connection or exchange bond through the switch layer 3. However, in any portion (high temperature region) where the temperature is raised beyond the Curie temperature Tc of the switch layer 3 due to irradiation of a reproducing laser beam 7, the switched connection between the recording layer 4 and the reproducing layer 2 is turned off, so that the magnetization of the relevant portion of the reproducing layer 2 is directionally turned to be the same as a reproducing magnetic field Hr applied from an external source. Consequently, the high temperature region serves as a mask to cover the record mark, whereby the data recorded in the recording layer 4 can be read out from the low temperature region within the beam spot 8. In this manner, the record mark can be read out from the region smaller than the beam spot diameter of the reproducing laser beam, so that it becomes possible to attain a resolution substantially equal to the value obtained by reproducing the information with a beam spot smaller in diameter than that of the reproducing laser beam.
FIG. 2 graphically shows the reproduced signal characteristics obtained by the use of the above MSR medium and a conventional ordinary magneto-optical recording medium, respectively. In the case of using the MSR medium, satisfactory reproduction characteristic can be attained even when the recording linear density is raised and the record marks are rendered smaller. The above-described method of reading out a record mark from a low temperature region while masking a high temperature region within a beam spot is termed an FAD (Front Aperture Detection) system.
Contrary to the above, a method of reading out a record mark from a high temperature region while masking a low temperature region within a beam spot is termed an RAD (Rear Aperture Detection) system, which is also capable of achieving high-resolution reproduction similarly to the FAD system. This RAD system can be realized by a magneto-optical disk of a dual film structure consisting of a magnetic reproducing layer and a magnetic recording layer. An initializing magnetic field is applied immediately before irradiation of a reproducing laser beam, and when a record mark has passed through the initializing magnetic field, the magnetization of merely the reproducing layer is directionally turned to be coincident with the initializing magnetic field.
In this stage of the operation, the recording layer holds the record mark. Immediately after application of the initializing magnetic field, the reproducing layer serves as a mask since the data in the recording layer is covered with the reproducing layer. And upon irradiation of a reproducing laser beam, the temperature of the reproducing layer serving as a mask is raised. When the strength of switched connection between the recording layer and the reproducing layer has exceeded the coercive force of the reproducing layer, the direction of magnetization of the recording layer is transferred to the reproducing layer. It signifies that the mask of the reproducing layer is removed from the high temperature region, whereby the record mark can be read out from the high temperature region.
Thus, in reproducing the MSR medium based on the FAD system, application of a reproducing magnetic field Hr is needed as described above. However, in an ordinary magneto-optical disk reproducing apparatus which is so constructed that no magnetic field is applied during a reproduction mode, there exists a problem that any information recorded on the MSR medium cannot be reproduced in the unmodified current construction of the known ordinary apparatus. FIG. 3 graphically shows the relationship between the peripheral velocity of the MSR medium and the power of a reproducing laser beam. This graph represents the minimum reproducing laser power required to obtain a C/N higher than 43 dB under the conditions including a mark length of 0.4 .mu.m with changes of the peripheral velocity.
The reproducing laser power for the ordinary magneto-optical disk ranges from 1.0 to 1.5 mW or so and has no dependency on the peripheral velocity, whereas the reproducing laser power for the MSR medium is considerably greater and its minimum required power varies depending on the peripheral velocity. Therefore, in the ordinary magneto-optical disk, no particular problem exists if a laser power of the above value is maintained to cause continuous emission of a laser diode. But in the MSR medium, there arises a serious problem that continuous maintenance of a high laser power during a reproduction mode deteriorates the service life of the laser diode.